1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and systems for achieving an enhanced operation of an expander using moveable inlet guide vanes at a compressor inlet.
2. Description of the Prior Art
Turboexpanders are widely used for industrial refrigeration, oil and gas processing and in low temperature processes. Turboexpanders are used, for example, to extract heavier hydrocarbon gases such as ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons, collectively referred to as natural gas liquids (NGL), from natural gas. A gas-liquid mixture resulting from an expansion of a raw gas in an expander is usually separated into a gas stream and a liquid stream. Most of the natural gas liquids are removed by outputing the liquid stream separately from the remaining gas stream, which is usually then compressed to be sent to downstream users.
FIG. 1 illustrates a conventional turboexpander-compressor system 100 in which a turboexpander 10 and a compressor 20 have impellers arranged on a same shaft 30. The turboexpander 10 is typically a centrifugal or axial flow expander inside which an incoming gas 40 is expanded. The gas expansion produces mechanical work causing a rotation of an expander impeller 50. The expanded gas is output as a gas flow 60. The gas flow 60 output from the turboexpander 10 may be input to the compressor 20 (i.e., the gas flow 70).
After an expansion (an isoentropic expansion may be used for calculation purposes) of the incoming gas 40 having a pressure p1 and a temperature T1 when entering the turboexpander 10, the gas flow 60 has a pressure p2 and a temperature T2 which are respectively lower than the pressure p1 and the temperature T1.
Since a compressor impeller 80 is mounted on the same shaft 30 as the expander impeller 50, the rotation of the expander impeller 50 causes the rotation of the compressor impeller 80. In this manner, the mechanical work produced in the turboexpander 10 is transferred to the compressor 20. The expander impeller 50, the compressor impeller 80 and the shaft 30 rotate at the same speed. The energy of the rotation of the compressor impeller 80 is used in the compressor 20 to compress the gas flow 70 input at a pressure p3 in the compressor 20. The compressor 20 outputs an output gas flow 90 having a pressure p4 higher than the pressure p3.
The pressure of the incoming gas 40 entering the turboexpander 10 is often controlled to be maintained around a design value. For example, a set of standard moveable input guide vanes (not shown in FIG. 1) may be used to control the pressure of the incoming gas 40 entering the turboexpander 10.
Ideally, at design conditions, the pressure p1 of the incoming gas 40, and the pressure p2 of the gas flow 60 output from the turboexpander 10 have predetermined values (i.e., within a range around the predetermined values). When the pressures p1 and p2 have the predetermined values, a speed u of the shaft is close to a design value. However, the turboexpander-compressor system at times functions in conditions different from the design conditions.
Generally, the turboexpander efficiency is related to a ratio of (i) the shaft speed u and (ii) the isoentropic enthalpy drop across the turboexpander 10. However, that a real transformation occurs in the turboexpander 10. The real transformation is determined when knowing a gas composition, the pressure p1 and the temperature T1 of the incoming gas 40, and the pressure p2 and the temperature T2 of the gas flow 60 output from the turboexpander 10. The isoentropic enthalpy drop across the turboexpander 10 can be calculated knowing the gas composition, the pressure p1, the temperature T1, and the pressure p2.
The compression in the compressor 20 passively affects the turboexpander efficiency by altering the speed u of the shaft 30. Therefore, in off-design conditions, the turboexpander efficiency is not optimized when a single parameter, the pressure p1 of the incoming gas 40, is adjusted. Being able to adjust only the pressure p1 of the incoming gas 40 limits an operator ability to optimize the turboexpander efficiency.
If no additional source of energy is used, the compression is a by-product of the expansion in the expander 10. The compression efficiency is determined by the pressure p3 of the gas input in the compressor 20, and a rotation speed of the compressor impeller, which is the same as the rotation speed u of the shaft 30.
In the conventional turboexpander-compressor system capable to adjust only the pressure p1 of the incoming gas 40, an operator has no leverage to fully control the rotating speed u of the shaft 30 for off-design conditions.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.